The present invention relates to a control for transferring an AC motor receiving power directly from a supply mains to receiving power from a solid-state inverter and for transferring an AC motor receiving power from a solid-state inverter to receiving power directly from a supply mains.
In some applications, such as gas turbine static starters, the purpose of a variable-speed solid-state power converter is to replace a diesel engine, which brings a turbine/generator up to a self-sustaining speed at which the turbine can then be fired and brought up to a speed at which the generator can be synchronized to and subsequently transferred to the power grid. The advantage of a solid-state power converter approach is that a single converter can be used to start one turbine, then be switched to start others.
In other applications, such as for variable speed fan and pump drives for example, it may be desirable to achieve variable speed operation with a power converter but transfer the motor to the supply mains for sustained operation at high speed. Often these drives are in such critical applications that in the event of a power converter outage, a backup system is designed to transfer the motor to the supply mains and control the process by some mechanical throttling means. In this case, since the power converter has failed, the transfer cannot be made bumpless, the power converter is switched out and the motor transferred to the line.
A bumpless transfer is defined as a transfer that does not cause an undesirable current transient in the motor, inverter, or supply mains. The normal, non-faulted power converter transfer is made bumpless by equalizing the motor voltage and phase to the supply mains and then in a make before break manner transfer the motor to the supply mains. The other half of the problem is to be able to transfer the motor back from the supply mains to the power converter, again, in a bumpless manner.
It is an object of the present invention to provide a control for transferring in a bumpless manner a motor from a power conditioner to a supply mains or from a supply mains to a power conditioner in a motor drive having a source converter and a load converter each using a phase-locked loop to control converter firings without requiring additional hardware.